Enhancing the Production of Maize

ABSTRACT

A maize plant and a method of enhancing maize production is provided. The maize plants, irrespective of their variety, exhibit at least two adjacent shortened internodes (defined as being equal to or less than twice the largest transverse dimension of the stalk above the ground) spacing at least two, ear nodes and a longer internode below the lowermost shortened internode provided that, in the event that the internodes are shortened down to ground level, such longer internode need be absent. Preferably the internode spacing is from less than about 50 mm to a maximum of 70 mm. The maize plants are preferably selected such that the ears develop substantially synchronously; the silks develop either at the same time as, or before, pollen is released by the tassels of the plants; and the maize plants are both of a dwarf maize type and a prolific plant type.

FIELD OF THE INVENTION

The invention relates to enhancing the production of maize; a method of developing modified maize plants with the aim of increasing grain yield capabilities and providing added production characteristics facilitating harvesting and other production activities.

BACKGROUND TO THE INVENTION

The goal of plant breeding is to combine in a single variety of hybrid, various desirable traits. For field crops, these traits may include resistance to diseases and insects, tolerance to heat and drought, reducing the time to crop maturity, greater yield, and better agronomic quality. With mechanical harvesting of many crops, uniformity of plant characteristics such as germination and stand establishment, growth rate, maturity, and plant and fruit height, is important.

Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self pollinated if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.

Plants that have been self-pollinated and selected for type for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two homozygous lines produces a uniform population of hybrid plants that may be heterozygous at a number of gene loci. A cross of two plants each heterozygous at a number of gene loci will produce a population of hybrid plants that differ genetically and will not be uniform.

Maize plants can be bred by both self-pollination and cross-pollination techniques. The maize has separate male and female flowers on the same plant located on the tassle and the ear respectively. Natural pollination occurs when wind blows pollen from the tassles to the silks that protrude from the tops of incipient ears.

The development of maize hybrids requires the development of homozygous inbred lines, the crossing of these lines, and the evaluation of crosses. Pedigree breeding and recurrent selection methods are used to develop inbred lines from breeding populations. Breeding programmes combine the genetic backgrounds from two or more inbred lines or various other broad-based sources into breeding pools from which new inbred lines are developed by selfing and selection of desired phenotypes. The new inbreds are crossed with other inbred lines and the hybrids from these crosses are evaluated to determine which of those have commercial potential.

Pedigree breeding starts with the crossing of two genotypes, each of which may have one or more desirable characteristics that is lacking in the other or which complement the other. If the two original parents do not provide all of the desired characteristics, other sources can be included in the breeding population. In the pedigree method, superior plants are selfed and selected in successive generations. In the succeeding generations the heterozygous condition gives way to homozygous lines as a result of self-pollination and selection. Typically in the pedigree method of breeding five or more generations of selfing and selection is practised: F1

F2; F2

F3; F3

F4; F4

F5; etc.

Backcrossing can be used to improve an inbred line. Backcrossing transfers a specific desirable trait from one inbred or source to an inbred lacking that trait. This can be accomplished for example by first crossing a superior inbred (A) (recurrent parent) to a donor inbred (non-recurrent parent), which carries the appropriate gene(s) for the trait in question. The progeny of this cross is then mated back to the superior recurrent parent (A) followed by selection in the resultant progeny for that desired trait to be transferred from the non-recurrent parent. After five or more backcross generations with selection for the desired trait, the progeny will be heterozygous for loci controlling the characteristics being transferred, but will be like the superior parent for most or almost all other genes. The last backcross generation would be selfed to give pure breeding progeny of the gene(s) being transferred.

A single cross maize variety is the cross of two inbred lines, each of which has a genotype which complements the genotype of the other. The hybrid progeny of the first generation is designated F1. In the development of hybrids only the F1 hybrid plants are sought. Preferred F1 hybrids are more vigorous than their inbred parents. This hybrid vigor, or heterosis, can be manifested in many polygenic traits, including increased vegitative growth and increased yield.

The development of hybrid maize involves three steps: (1) the selection of plants from various germplasm pools; (2) the selfing of the selected plants for several generations to produce a series of inbred lines, which, although different from each other, breed true and are highly uniform; and (3) the crossing of selected inbred lines with unrelated inbred lines to produce the hybrid progeny (F1). An important consequence of the homozygosity and homogeneity of the inbred lines is that the hybrid between any two inbreds will always be the same. Once the inbreds that give a superior hybrid have been identified, the hybrid seed can be reproduced indefinitely as long as the homogeneity of the inbred lines is maintained.

A single cross hybrid is produced when two inbred lines are crossed to produce the F1 progeny. A double cross hybrid is produced from four inbred lines crossed in pairs (A×B and C×D) and then the two F1 hybrids are crossed again (A×B and C×D). Much of the hybrid vigor exhibited by the F1 hybrids is lost in the next generation (F2). Consequently, seed from hybrid varieties is not used for planting stock.

Maize is an important and valuable field crop. Thus, a continuing goal of plant breeders is to develop high yielding maize hybrids that are agronomically sound based on stable inbred lines. The reasons for this goal are obvious: to maximise the amount of grain produced with the inputs used and minimise susceptibility of the crop to environmental stresses. To accomplish that goal, the maize breeder must select and develop superior inbred parental lines for producing hybrids. This requires identification and selection of genetically unique individuals which in a segregated population occur as the result of a combination of crossover events plus the independent assortment of specific combinations of alleles at many gene loci which results in specific genotypes. Based on the number of segregating genes, the frequency of occurrence of an individual with a specific genotype is less than 1 in 10,000. Thus, even if the entire genotype of the parents has been characterised and the desired genotype is known, only a few if any individuals having the desired genotype may be found in a large F2 or F0 population. Typically, however, the genotype of neither of the parents nor the desired genotype is known in any detail.

Several different strategies have been used to increase the grain yield of maize. The most obvious and generally utilised approach is to select for increased yield per se. Another approach is to try to preserve the inherent yield potential by reducing losses that occur due to disease and insect pests and following exposure to environmental stress. Thus, in many commercial programmes, breeders select for disease and insect resistance and tolerance of drought stress.

Still another approach is the use of breeding material with the capability to produce more kernels under favourable conditions. For example, some varieties express prolificacy or the capacity to form a second grain filled ear. Other varieties can express “ear-flex”; the capacity to develop an ear with more and/or larger kernels. Many commercial hybrids express one or the other of these traits.

Yet another approach to increasing yields has been to increase the number of plants per unit area, typically per hectare or acre, or the “plant density”. It has been stated that in the 1930s, maize farmers in the USA planted 10,000 plants per acre whereas by 1998, the farmers were planting 20,000 to 30,000 plants per acre. With increasing plant density maize plants tend to grow taller and become more susceptible to lodging. Thus, to develop commercial products adapted to higher planting rates, breeders have selected for resistance to stalk and root lodging.

Maize has shown a limited ability to increase yields of both dry matter and grain as plant densities are increased. Mock & Pearce (1975) described an optimum environment to produce maximum yields as including, among other factors, high plant densities and narrow rows. High plant densities and narrow rows permit increased leaf area index (LAI (Leaf Area Index)=leaf area per unit land area) allowing interception of more of the light energy reaching the earths surface.

Presently available maize plant morphologies (prolific phenotypes) often enable a maize plant to produce a second ear of harvestable grain but only if plant densities are below normal for typical growing conditions. Earley et al (1974) noted the yield of grain per plant was not limited by the lack of potential ears but by the failure of one or more of the earshoots to develop into sizeable ears. Results of experiments by Harris et al (1976) indicated that the certain lower earshoots abort because they reach the silking stage in poor synchrony with upper ear shoots. This was confirmed by studies by Sorrels et al (1979). As plant densities are increased, the incidence of plants with two ears becomes progressively lower. Prine (1971) concluded that under high plant densities competition for light during the critical silking period resulted in sizeable reductions in grain yield. Moreover, with increasing plant densities, the incidence of plants with multiple ears decreases more rapidly than the total vegetative matter per plant. Thus, obtaining a morphology which routinely produces multiple ears with harvestable grain, on a plant with equal or less total dry matter yield, would be an advantageous alternative for increasing yield. However, under very high plant densities, most of the plants are barren even though the total dry matter yield per unit area of land increases with increasing plant densities. Although grain yield is reduced at very high populations, the result is encouraging. This population response of the plant suggests that with appropriate technology to enable more efficient conversion of photosynthate into grain, the grain yielding potential of maize on a per acre basis could be further increased as well.

The productivity of rice and wheat has been increased significantly by introgressing dwarfism into the plant morphologies. Reducing plant stature reduced dry matter yield on a per plant basis, but the ratio of grain yield to total dry matter yield increased. The shorter plant type of these dwarf morphologies exhibits less lodging under high fertility levels. Hence, these rice and wheat morphologies were able to respond to higher fertilities by increasing grain yield per se. The ability of these varieties to increase the ratio of harvestable grain to total plant dry matter without a corresponding tendency to lodge at higher yield levels, produced the dramatic yield increases known as the “green revolution” in the late 1960s and early 1970s.

Attempts to improve the yield of maize by applying the dwarfing strategy used so effectively in wheat and rice have not to date been successful. A widely used gene in maize is the brachytic 2 mutant. The lower internodes of brachytic 2 dwarfs are much shorter than in normal maize. Leng & Ross (1979) found that at comparable planting rates brachytic 2 dwarf hybrids showed better standability than their normal counterparts but yielded less. Subsequently, Pendelton & Sief (1961) found that the yield deficiency of brachytic 2 dwarfs could not be overcome by closer row spacings or higher plant populations. A more recent effort by Castiglioni et al (1991) studied the effects of the brachytic 2 gene following introduction into seven maize varieties. The introduction of the gene significantly reduced plant and ear height, plant lodging also decreased significantly. However, as with previous studies, grain yields declined compared to normal counterparts, and there was a measured reduction in the degree of prolificacy.

Research by plant physiologists has identified the magnitude of the supply of photosynthate available to convert into grain (also called “source” capacity), and the capacity to convert that supply into grain (also called “sink” capacity), as potentially limiting factors in maize yield. Tollenar (1977) summarised those reports by concluding that grain yield is limited by sink size in most temperate and subtropical maize growing environments, the exception being the northern areas of North America where the source appears to be limiting. The source limitations could be overcome by increasing LAI through high-density plantings to the point when sink size ultimately becomes the limiting factor in grain yield, if not for the fact that eventually high plant density suppresses expression of prolificacy and reduces grain yield. Anderson et al (1984) conducting N rate studies confirm the results of Harris et al (1976) to the effect that reproductive sink size limits the yield of non-prolific hybrids. When sink size is the limiting factor, increasing the number of potential energy sinks (ear sites) could be achieved through improved multiple ear capability (prolificacy). Traditionally, Tollenar (1977) noted that increasing the amount of photosynthate to the ear during flowering would also increase yields. Thus, improved multiple ear formation and increased available photosynthate during flowering might alter the source-sink relationship and improve the proportion of photosynthate available to produce grain. This may be accomplished by breeding for dwarf maize plants, as was done with wheat and rice.

OBJECT OF THE INVENTION

It is, accordingly, an object of this invention to provide, in the first place, modified maize plants that enable increased yields to be achieved and, in the second place, to provide a method of developing such modified corn plants.

DEFINITIONS

-   Acropetally. Development of organs in succession towards the apex,     the oldest at the base, the youngest at the tip, for example the     leaves on a shoot. -   Assimilate. The product of photosynthesis used by a plant to produce     grain. -   Basipetal. The development of organs in succession towards the base,     the oldest at the apex, the youngest at the base. -   Canopy The state at which the plants cover the whole area and     intercept virtually all the light. -   Internode. The part of the plant stem between the two successive     nodes (q.v.). -   Ear node. A node at which an ear develops. -   FIL. Floral internode length -   F1. First generation cross. -   F2 second generation from a self pollinated cross. -   F3. Third generation from a cross which results from the self     pollination of the F2, and so on. -   LAI Leaf Area Index. The area of leaf per unit area. -   Lodge The falling over of plants. -   Node. Part of the plant stem where one or more leaves arise. One     leaf in the case of maize. -   MEWI Multiple Ear grain Weight Index. -   Morphology. Form. -   Protogyny. The female flower (the ear in maize) exerting silks     before the male flower (the tassle) sheds pollen. -   Sink The ability of the grain to accommodate the assimilate     (photosynthate). -   Shortened (or short) Internode An internode distance that is less     than or equal to twice the largest diameter (transverse dimension)     of the stalk just above ground level -   Source. Supply to the grain of the products of photosynthesis     (assimilate or photosynthate). -   Prolificacy the ability of the maize plants to produce grain on more     than one ear.

SUMMARY OF THE INVENTION

In accordance with one aspect of this invention there is provided a method of enhancing the yield per unit area of a maize producing facility, the method comprising growing a maize type in such facility wherein the maize type is selected such that at least the majority of the maize plants develop at least two adjacent shortened internodes spacing at least two ear nodes and a longer internode below the lowermost shortened internode provided that, in the event that the internodes are shortened down to ground level, such longer internode may be absent.

Further features of this aspect of the invention provide for the maize to be selected so that each maize plant develops two, three, four or more ear nodes that are spaced by an internode spacing sufficiently small, typically from less than about 50 mm to a maximum of 70 mm, to ensure that the ears develop substantially synchronously; for the maize to be selected such that the silks develop either at the same time, or before, pollen is released by the tassels of the plants; for the maize to be selected such that the tassels are close to the silks; for the maize type to be a dwarf maize; for the maize to be a prolific plant type; and for the maize to be grown in high-density populations selected, with particular reference to maximising the use of available photosynthate over the growing season.

In accordance with a second aspect of the invention there is provided a maize plant that, irrespective of its variety, exhibits at least two adjacent shortened internodes spacing at least two ear nodes and a longer internode below the lowermost shortened internode provided that, in the event that the internodes are shortened down to ground level, such longer internode may be absent.

Further features of this aspect of the invention provide for the maize plant to develop two, three, four or more ear nodes that are spaced by an internode spacing sufficiently small, typically from less than about 50 mm to a maximum of 70 mm; for the maize plants to be such that the ears develop substantially synchronously; for the maize plants to be such that the silks develop either at the same time as, or before, pollen is released by the tassels of the plants; for the maize to be selected such that the tassels are close to the silks; for the maize plant to be a dwarf maize type; and for the maize to be a prolific plant type.

The maize morphology of the present invention includes at least two shortened internodes with each internode being bounded by an ear node. The three or more ears developing from the ear nodes proximate to the shortened internodes develop normally and relatively synchronously with each other. The shortened internodes of this maize phenotype are the result of one or more genetic loci. The internodes underneath the lowest grain bearing ear and above the apical ear, may be greater in length than those bounded by the ear nodes unless the internodes are shortened down to ground level.

The instant invention also encompasses the methods and process of breeding or developing maize plants with the attributes described herein. Both inbred lines, hybrids and any other maize plants with these traits are within the scope of this invention. Additionally, the present invention further encompasses regenerable plant materials therefrom and the processes by which plants may be regenerated from tissue culture and the tissue culture arising from regenerable tissues of maize plants of the present invention.

The instant invention also includes inbred lines to which the trait of this invention has been transferred either by conventional breeding strategies including but not limited to the pedigree breeding, recurrent selection breeding or backcrossing or by nonconventional biotechnology or genetic engineering strategies. Also included are hybrids made by crossing one or more inbred lines containing the trait of this invention.

A further embodiment of this invention is maize containing the trait of this invention, according to methods described from diverse germplasm sources containing the appropriate gene(s), and especially maize lines designated AC 101, APN303, APN304, APN305, APN306. APN301, APN312 and APN313. The maize lines designated APN312 and APN313 had shortened internodes down to the ground level whilst the others had a longer internode below the lowermost shortened internode.

A further feature of this invention is in the maize inbred or hybrids characterised by the trait of this invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The method of breeding maize provided by the invention makes it possible to increase the number of grain-bearing ears that individual plants produce, that is, to increase the sink capacity of the maize plant. This is achieved by the selection of plants with very short internodes between the nodes that produce ear shoots. When the internode is reduced below a critical length, the ear shoots at either end of the short internodes develop synchronously and both produce grain. If the plant is bred with one short internode below the apical ear, followed by a long internode, then two grain bearing ears are produced. If the plant is bred with two short internodes, one below the other and followed by a long internode then three grain bearing ears will develop. If the plant is bred to have three such short internodes followed by a long internode then four grain bearing ears. In this way grain bearing ears can continue to be added down to the node below ground level provided that plant has the photosynthetic capacity (source) to fill them and no other limiting factor (usually environmental) starts to limit the supply of photosynthate.

The plants that are able to produce grain bearing ears down to the node below ground level at relatively low population densities are highly desirable because they are very short and have a very high photosynthetic capacity. These indicate a strong potential and suitability to produce grain on at least the top ear at extremely high population densities. The ears will produce grain provided that the plant has the photosynthetic capacity to fill them. In this way grain bearing ears can continue to be added up to the limit of the plant's photosynthetic capacity (source) or until some other limiting factor, usually environmental, starts to limit the production of photosynthate in the maize plant.

As population density increases mutual shading results in a reduction in the amount of solar radiation received by individual plants. Less solar radiation means less photosynthesis thus lower grain production by individual plants. Grain production is reduced from the lowest ear upwards. As population density increases a point is reached where only one ear, the top ear, produces grain. At even higher population density no grain at all is produced. Even though the individual plants produce less grain where one ear per plant sets grain, the yield per unit area is greater because of the greatly increased number of plants per unit area. All things being equal, the most efficient plants, namely those that produce the most ears and therefore grain at lower population densities, can be expected to produce at least one ear per plant at the highest population densities where less efficient plants produce no ears at all.

The present invention emerged from a maize breeding programme initiated to increase the productivity of the crop. In looking at other crops, like grain sorghum, wheat and rice, the productivity was increased by introducing dwarfism into improved plant morphologies. The first approach was to select for a dwarf plant type in combination with selection for tillering and prolificacy. Two inbred lines, 85-W-Dw selected out of the Zulu landrace and 84-14 selected out of what might possibly have been the Mostert open pollinated variety. Both inbreds had a dwarf stature and 84-14 formed tillers. The F1 hybrid between these inbreds was also a dwarf, but the yield was poor.

To increase the vigor and strengthen the heterotic response the 85-W-Dw and 84-14 inbreds were crossed to several inbred lines of different heterotic groups, and the resulting F1 hybrids were selfed. Among the segregating progeny dwarf plants were selected and further inbred. One such cross was 136×85-W-Dw. Among the progeny of this particular cross were some plants which produced two well-developed ear shoots that tended to produce grain on the top ear only. By pollinating the second ear only several dwarf prolific inbred lines were bred which exerted their silks before pollen shed (protogyny). Although prolific plants were selected in each generation there was always a proportion of the progeny which only produced one ear. In other words the inbred lines did not become true breeding for prolificacy.

While inspecting the yield trial it was noticed that one plant in a prolific hybrid variety belonging to Pioneer (now Pannar) had a very short internode between its ears. None of the other plants in this hybrid had such a short internode between the ears. Examination of prolific inbred lines revealed that, in general, the prolific plants had slightly shorter internodes between the ear shoots than the plants in the same lines on which only the top ear produce grain. Further, the internode(s) between the ears of prolific plants were shorter than the internodes immediately above and below the ears. Based on this observation subsequent line development was based on selection for short internodes between the ears of prolific plant types. The result was the development of a series of inbred lines from different backgrounds characterised by dwarf stature and prolific plant type with short internodes between the ears that silk synchronously. The following lines AC101 and APN305 exemplify this plant type and are part of the present invention.

The critical characteristic of the present invention is the reduction of internode length between the nodes which develop ear shoots or buds (ie ear nodes). When the internode length is reduced to below a certain critical distance, the proximal ears develop synchronously as if the control system of the plant treated these multiple ear nodes as a single unit. Thus, if each of the internodes between the top three or four ear nodes is less than a certain critical distance, the ear shoot primordia adjacent to these ear nodes will develop simultaneously. Moreover, if the internode lengths are shortened to between 50 and 70 mm then the proximal developing ears may have virtually equal access to the available photosynthate. Regardless of the mechanism of the present invention, up to seven or eight ears may develop on plants, whereas otherwise only the top one or two shoots would develop. In addition, the development of these ears is more synchronised. If the supply of photosynthate is adequate, all the ear shoots will fill and produce grain at relatively low population densities.

The critical internode length can be somewhat affected by the vigor or overall height of the plant. The critical internode length required for a plant whose vigor has been reduced by inbreeding is 50 mm or less, whilst the critical internode length for a vigorous and taller hybrid resulting from the cross of two such inbred plants may be greater than 70 mm.

The impact of short internodes on ear formation is unique and in contrast to a number of findings of other researchers. Torregroza et al (1988) looked at the impact that breeding for prolificacy had on internode pattern and found that prolific populations had more and longer internodes than non-prolific populations. They also noted that the increase in internodes number occurred below the ear increasing both ear height and plant height which is not advantageous in a commercial product. Sorrells et al (1979), in studying the inheritance of prolificacy, noted that there were a few correlations observed between internode lengths and either ear number or MEWI (an index of multiple ear grain weight). They did find that the number of nodes above and below the first ear were highly correlated to ear number and MEWI.

Harris et al (1976) postulated a genetic model for prolific potential in maize that included internode length, however they did not indicate whether they believed shorter or longer internodes were involved. In earlier studies Stemer et al (1969) found that stalk internode elongation and auxiliary bud activity proceed acropetally and they indicated the activities may be related. However, they concluded distinct ear formation proceeds basipetally and can hardly be related to individual internode elongation. Scott & Campbell (1969) studied the internode lengths in brachytic 2 germplasm. They found internode lengths for dwarf inbred ranged from 63 mm to 288 mm for all internodes except the first above the ear which was 134 mm, and the first below the ear which was 55 mm. The internode lengths for dwarf hybrids ranged from 105 mm to 144 mm for all internodes except the first above the ear which was 160 mm, and the first below the ear which was 76 mm.

One feature of the present invention is that dwarfing results in a smaller plant having less tissue. Thus, less energy is required to grow and sustain the reduced amount of tissue for a smaller plant versus larger plants. These lower energy requirements should enable more photosynthate to be used for grain production. Moreover, a shorter stalk indicates the total volume of the plant is smaller than otherwise. Another advantage of the smaller plant volume is the resulting higher concentrations of photosynthate expected during the critical flowering period. The higher concentrations of photosynthate would promote the development of a number of potential ear shoots that can develop into mature ears bearing harvestable grain. The dwarfing mechanism of the present invention is distinct from and does not have the shortcomings previously referenced regarding the use of brachytic 2 dwarf gene. Specifically, brachytic 2 dwarf plants do not have the favourable pattern of short and long internodes that characterise the present invention.

It is known that plants with one or more fully developed ears are more susceptible to lodging than plants without developed ears. It is generally believed that increased competition between developing ears and the stalk for photosynthate causes greater reallocation of stalk carbohydrates to the grain sink. This may predispose the plant to root senescence followed by root rot and stalk rot, Dodd (1977) and Duvick (1974). It is also known that taller plants and plants with greater ear heights are more susceptible to lodging because of the physical leverage of the wind during storms in comparison to shorter plants and plants with lower ear heights. Thus, the very short stalk length above the ear and the decreased ear heights and often thicker, stronger stalks of dwarf selections of the maize plants of the present invention are less prone to lodging than the current morphologies in commercial use. Therefore another desirable, but not essential feature of the improved maize morphology of the present invention, is that it includes a lower ear height and stronger stalks than the current commercial maize morphologies.

To ensure the greatest possible amount of photosynthetic activity, the leaves should be upright in such a way as to allow for efficient light penetration into the leaf canopy. Mock & Pearce (1975) indicated leaf orientation (ie angle of the leaf blades relative to the stem) to be of prime importance with respect to light interception by plants grown at high plant densities. It is especially advantageous, but not essential to the improved maize morphology of the present invention that the leaf orientation be upright so that as much sunlight as possible penetrates to the photosynthetic tissues proximate to the developing ears. Long broad lower leaves would also be beneficial but not essential to the present invention in that they would increase leaf area and enable the plants to canopy earlier and so increase productivity.

Another advantageous, but not essential feature of the present invention is an improved maize morphology via a reduction in tassel mass. Mock & Pearce (1975) included tassel size as an important trait of their maize ideotype. They suggested that small tassles should reduce both the competitive ability of the tassel and the shading of the upper leaf layers. Smaller tassels would reduce the photosynthate required for pollen growth and development and thus free more photosynthate for grain production. Moreover, less shading of the underlying canopy also occurs, thereby further increasing the photosynthetic potential of leaves proximate to the developing earshoots.

During silking, each potential kernel (ie ovule) on an ear produces a tube-like structure called a silk. The silk grows until it emerges from the husks surrounding the ear tip. Pollination occurs when silks protruding from the earshoots intercept pollen grains. Intercepted pollen grains germinate on the silk, each sending out a tube that grows down the centre of each silk towards the ovule. When the tube finally enters the embryo sac, it ruptures releasing two sperm, which fuse with the egg to initiate development of both the embryo and kernel. In order for a given ear to have a full compliment of grain, each ovule must be fertilised. In order for each ovule to be fertilised, viable pollen must be present during or shortly after the emergence of the silks.

In current maize germplasm and morphologies, silk emergence normally occurs after the maize plant begins to shed pollen. If stress reaches a certain magnitude, such as that produced by drought or increased population density, then silking is further delayed. Extended delay can cause silking to occur at a time when the amount of viable pollen is either greatly reduced or no longer present. In such cases, few or no kernels are formed on these ears and grain yield is severely reduced.

Plants having silks which normally begin to emerge a few days before pollen shed are more resistant to both drought and higher population densities compared to plants which silk after the onset of pollen shed. During times of stress, the silks of these early silking plants are delayed, a silk emergence is still more fully synchronised with pollen shed. Therefore even during times of stress there is sufficient viable pollen to effect fertilisation. Hence, ears with full compliments of grain are enabled. This presents another desirable feature of the present invention for the maize plant to have the capability to silk before the onset of pollen shed.

Another important feature of the present invention is tolerance to higher populations. It is well established in the art of maize production that hybrids of prolific tendencies respond more favourably to high-density planting than single eared plant types. Data from Bauman (1959) confirmed the ability of prolific hybrids to resist barrenness at high plant densities or under other stress environments. Normal single-eared hybrids only adjust to higher plant densities, in addition to increased barrenness, by reduction in ear size. In contrast, prolific hybrids adjust to higher densities only by reduction in ear number with no barrenness and relatively constant ear size. This was confirmed by Hallauer & Troyer (1972). Thus, the maize morphologies of the present invention which are able to produce three or more grain bearing ears per plant, and higher plant densities, confer even greater yield responses. Other beneficial but non essential attributes of the present invention are a high resistance to disease and a large fibrous root system with a large number of hair roots which means that the surface area in contact with the soil is much larger than a plant which has a smaller root system and few hair roots. This could be important under low moisture conditions as a larger surface area increases the ability of the plant to extract moisture from the soil.

Line development continued on a number of materials. The greatest success was obtained where pollinations were made with bulked pollen. This allowed for a large number of combinations of genes to take place.

Examples of successful pedigrees.

Example 1

A228N×B254W Dw Lns Bk

A228N is a short line. B254W Dw Lns Bk denotes the bulking of the pollen of these polygenic dwarf lines. The resulting F1 cross of these two lines had a height of 2.1 m. Because the cross produced taller offspring, each line apparently carried genes that were dominant over the recessive genes for short internodes carried by the other side.

Pollen of the tillering F1 plants was bulked and the tillering F1 plants were pollinated with this bulked pollen. This method of intermating siblings allowed for the genes for all of the lines to recombine. S0(A228N×B254W Dw Lns Bk)−1Til×Til Bk

Short plants were selected out of this segregated population and self pollinated. S1[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9

This plant set grain on two ears, with a third ear with no seed.

The FIL:60 (FIL=floral internode length=length of internode between ears in mm) S2[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8

This plant had two grain bearing ears and a third ear shoot that was sterile.

F1L: 30-70 S3[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8−6

A threshold of expression was apparently crossed in this generation as it segregated for multiple ears. This plant had four well-developed ears. Seed set on the upper three ears were scattered. The first and third ears set seed while the second did not, even though it was a well-developed full-sized ear. The fourth ear, separated from the third by a longer internodal distance, did not produce seed. It was believed the dramatic increase in sink capacity in this generation created a demand for increased photosynthetic capacity (source). The level of N fertiliser was increased for the next generation.

FIL: 40-70-145 S4[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8−6−15

This selection had four grain bearing ears with very good seed set. The size of the ears was fairly uniform and about the same size as one of the original parent inbreds, A228N.

FIL: 25-25-35 S5[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8−6−15−22

This was the best four-eared selection of this generation and had good seed set. However, the seed was rather small indicating that the source capacity of the plant was likely to be at its limit. Almost all of the siblings of this population were three-eared and had good seed set on the three ears.

FIL: 20-30-27

Example 2

S0{[D940Y−1×(B254W×85-W-Dw)−1−3−390C]×[(B254W×85-W-Dw)Dw Lns Bk]¹}

The first cross [D940Y−1×(B254W×85-W-Dw)−1−3−390C] was tall, meaning that two lines were heterozygous for genes controlling internode length. Segregation for short internodes was expected. S1{[D940Y−1×(B254W×85-W-Dw)−1−3−390C]×[(B254W×85-W-Dw)Dw Lns Bk]¹}−8

This selection had two ears and a small tassel. S2{[D940Y−1×(B254W×85-W-Dw)−1−3−390C]×[(B254W×85-W-Dw)Dw Lns Bk]¹}−8−10

This selection was prolific with two ears.

FIL:50 S3{[D940Y−1×(B254W×85-W-Dw)−1−3−390C]×[(B254W×85-W-Dw)Dw Lns Bk]¹}−8−10−22

This selection had two grain-bearing ears with an internode of 50 mm between them. This appears to be the threshold necessary for the production of multiple ears. Its progeny segregated for multiple ears. Many of the progeny also had very poor pollen production. It was necessary to select for good pollen as well as for short internode length.

FIL: 50 S4{[D940Y−1×(B254W×85-W-Dw)−1−3−390C]×[(B254W×85-W-Dw)Dw Lns Bk]¹}−8−10−22−12

This plant set seed on three ears. The silks emerge before anthesis. It had a very small tassel but with very good pollen production.

FIL 35-40 S5{[D940Y−1×(B254W×85-W-Dw)−1−3−390C]×[(B254W×85-W-Dw)Dw Lns Bk]¹}−8−10−22−12−7

Although this selection had a very desirable configuration, namely, short internodes down to ground level, only the top ear set seed. The fact that a plant in the next generation set 4 ears probably indicates that environmental factors were responsible for only one ear being set and not an inherent lack of vigour. By using breeding methods that lead to a slower rate of inbreeding it is possible to produce more vigorous inbreds which set seed on a larger number of ears.

FIL: 32-39-29-31-26-34 S6{[D940Y−1×(B254W×85-W-Dw)−1−3−390C]×[(B254W×85-W-Dw)Dw Lns Bk]¹}−8−10−22−12−7−4

S6 This plant set seed on the top four ears. It appears that due to a longer internode below the fifth ear shoot the ears shoots below it were at a disadvantage in competing for photosynthate. This means that under very good environmental conditions there could be a shortage of sink capacity due to the long internode inhibiting the development of ear shoots below it. Under poorer conditions it is possible that there might be an advantage in limiting the size of the sink.

FIL: 45-45-55-45

Example 3

S0(A228N×B254W Dw Lns Bk)−1Til×Til Bk

A228N (a short line) was pollinated with bulked pollen of B254W.Dw (a cross of two polygenic dwarf lines). This produced a tall high yielding hybrid. The fact that it was tall indicated the parents had different recessive genes for dwarf and it was possible for recombination of genes for short internodes to take place. Some of the plants had tillers. The pollen of the tillering plants was bulked and used to pollinated the same tillering plants. Short prolific and semi-prolific plants were selected out of the segregating population. S1[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9

One of this plants progeny had an exceptionally short floral internode (30 mm).

FIL:60 S2[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8

This is the point when the threshold had been crossed and the progeny of this plant segregated for multiple ears.

F30-70 S3[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8−6 FIL:40-70-145 S4[(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8−6−7

This plant set seed on four ears.

FIL: 20-30-60-75 S4 [(A228N×B254W Dw Lns Bk)−1Til×Til Bk]−9−8−6−7−55

This plant set seed on three ears.

FIL: 25-20-40.

Example 4

In order to produce dwarf lines with short internodes derived from the inbred B73, it was pollinated with bulked pollen of B254W dwarf lines. Selections were made in the segregating population which was produced when the cross was self pollinated.

This resulted in a number of lines such as: (B73×B254W Dw Lns Bk)−1−970C−14−7−7

Although these lines were short none of them had short internodes. It was decided to pollinate one of these lines with bulked pollen of selections out of all its sister lines as follows: (B73×B254W Dw Lns Bk)−1−970C−14×(B73×B254W Dw Lns Bk) selections bulk

This created a population which was designated EF8. The reason for doing this was in order to allow the genes to recombine so as to obtain plants with short internodes. However, no short internode plants segregated out in this population. It was then decided to introduce the required genes from a line that had short internodes. F1[(A228N×B254W Dw Lns Bk)−9−8−6−7−Bulk]×EF8−251D BC1{[(A228N×B254W Dw Lns Bk)−9−8−6−7−Bulk]×EF8−251D}− Bulk×(B73×B254W Dw Lns Bk)−1−970C−14−7−7 3 silks Bulked

The F1 population was then backcrossed to

-   -   (B73×B254W Dw Lns Bk)−1−970C−14−7−7 by pollinating it with         bulked pollen of selections in the line which had three ear         shoots with silks.     -   BC1,S1 In the backcross population plant 723D was selected and         self pollinated.     -   BC!,S2         -   723D-770D         -   FIL: 65-60-70

The selection 723D-770D was a vigorous plant which had short internodes and set seed on three ears.

-   -   BC!,S3 723D-770D-851D

Plant 851D was a very short selection.

-   -   BC!,S3,Sib1 723D-770D-851D-80E×723D-770D-851D-4E     -   BC1, S4 723D-770D-851D-4E     -   BC1, S4 723D-770D-851D-80E

The selections 4E and 80E were selfed and crossed with each other.

-   -   BC!,S3,Sib 1 (723D-770D-851 D-80E×723D-770D-851 D-4E)-25F

The selection 25F was far more vigorous and set more ears than the selfed progeny of either 80E or 4E. This demonstrates the importance of using methods of breeding such as populations and sibling to produce vigorous inbreds that are able to set seed on the multiple ear shoots that are produced.

The plant 25F has short internodes right down to ground level. This is preferred configuration as:

1. It favours the production of multiple ears at low population density (40,000 plants per Hectare) provided that the plant is sufficiently vigorous and that environmental conditions are favourable.

2. With short internodes down to ground level this means that the plant is very short in stature and

-   -   a. That less photosynthate is required to produce the stalk. As         a result more photosythate is available for the production of         grain.     -   b. That the stalk contains very much less protein than is         contained in the stalks of taller plants. Protein requires the         expenditure of a relatively large amount of energy to maintain         it. The photosynthate saved in this respect is also available         for the production of more grain.     -   c. The shorter the plant the less likely the plant is to topple         over. This is very important in the case of very high density         plantings.         3. The plant does not have a shortage of sink capacity.

REFERENCES

-   Anderson, E. L., E. J. Kamprath, R. H. Moll, and W. A.     Jackson (1984) Effect of N Fertilization on Silk Synchrony, Ear     Number, and Growth of Semiprolific Genotypes Crop Science 24:663-666 -   Bauman, Loyal F. (1959) Relative Yields of First (Apical) and Second     Ears of Semi-Prolific Southern Corn Hybrids Agronomy Journal 52:     220-222 -   Casliglioni, V., J. Silva, C. Cruz, L. Saraiva, and C. Silva (1991)     Effects of the Introduction of the Brachytic-2 Gene in Seven     Varieties of Corn Revista Ceres 38(216): 81-93 -   Conger, B. V., F. J. Novak, R. Afza, and K Erdelsky (1987) Somatic     Embryogenesis from Cultured Leaf Segments of Zea Mays Plant Cell     Reports, 6: 345-347 -   Dodd, J. L. (1977) A Photosynthetic Stress-Translocation Balance     Concept of Stalk Rot Proceedings of the 32nd Annual Corn Sorghum     Research Conference. p 122-130 -   Duncan, D. R., M. E. Willilams, B. E. Zohr, and J. M. Widholm (1985)     The Production of Callus Capable of Plant Regeneration from Immature     Embryos of Numerous Zea Mays genetypes Planta 165: 322-332 -   Duvick, D. N. (1974) Continuous Backcrossing to Transfer Prolificacy     to a Single-Eared Inbred Line of Maize Crop Science 14: 69-71 -   Earley, E. B., J. C. Lyons, E. Inselberg, R. H. Maier, and E. R.     Leng (1974) Earshoot Development of Midwest Dent Corn Illinois     Experiment Station Bulletin #747 -   Green & Rhodes (1982) Plant Regeneration in Tissue Culture of Maize     Maize for Biological Research Plant Molecular Biology Association,     Charlottesville, Va. 1982 at 367-372 -   Hallauer, A. R. and A. F. Troyer (1972) Prolific Corn Hybrids and     Minimizing Risk of Stress Proceedings of the 27th Annual Corn     Sorghum Research Conference. p. 140-149 -   Harris, R. E., R. H. Moll, and C. W. Stuber (1976) Control and     Inheritance of Prolificacy in Maize Crop Science 16: 843-850 -   Leng, E. R. and G. L. Ross (1960) 1959 Performance of Commercial     Hybrids in Illinois Illinois Agr. Exp. Sta. Bul. 651. 1960 -   Loomis, R. S. and W. A. Williams (1963) Maximum Crop Productivity:     An Estimate Crop Science 3: 67-72 -   Mock, J. J. and R. B. Pearce (1975) An Ideotype of Maize Euphytica     24: 613-623 -   Pendelton, J. W. and T. D. Seif (1961) Plant Population and Row     Spacing with Brachytic 2 Dwarf Corn Corn Crop Science 1: 433-435 -   Prine, G. M. (1971) A Critical Period for Ear Development in Maize     Crop Science 11: 782-786 -   Rao, K. V., P. Suprasanna, and G. M. Reddy (1986) Somatic     Embryogenesis in Glume Callus Cultures Maize Genetics Cooperation     Newsletter 60: 64-65 -   Scott, Gene E. and C. M. Campbell (1969) Internode Length in Normal     and Brachytic-2 Maize Inbreds and Single Crosses Crop Science 9:     293-295 -   Siemer, F. G., F. R. Leng and O. T. Bonnett (1969) Timing and     Correlation of Major Developmental Events in Maize Agronomy Journal     61: 14-17 -   Songstad, David D., David R. Duncan, and Jack M. Widholm (1988)     Effect of 1-aminocyclopropane-1-carboxylic acid, silver nitrate, and     norbornadiene on Plant Regeneration from Maize Callus Cultures Plant     Cell Reports 7: 262-265 -   Sorrels, M. E., J. H. Lonnquist, and R. E. Harris (1979) Inheritance     of Prolificacy in Maize Crop Science 19: 301-306 -   Tollenaar, M. (1977) Sink-Source Relationships During Reproductive     Development in Maize: A Review Maydica 22: 49-75 -   Torregroza, M., O. Martinez, I. Pulido, and G. Jiminez (1989)     Divergent Mass Selection for Prolificacy in Maize and Its Effect on     the Internode Pattern Agronomia colombiana 5: 53-59 -   Williams, W. A., R. S. Loomis, and C. R. Lepley (1965) Vegetative     Growth of Corn as Affected by Population Density I. Productivity in     Relation to Interception of Solar Radiation Crop Science 5: 211-215 -   Williams, W. A., R. S. Loomis, and C. R. Lepley (1965) Vegetative     Growth of Corn as Affected by Population Density II. Components of     growth, Net at Assimilation Rate and Leaf Area Index Crop Science 5:     215-219. 

1. A method of enhancing the yield per unit area of a maize producing facility, the method comprising growing a maize type in such facility wherein the maize type is selected such that at least the majority of the maize plants develop at least two adjacent shortened internodes spacing at least two ear nodes and a longer internode below the lowermost shortened internode provided that, in the event that the internodes are shortened down to ground level, such longer internode may be absent.
 2. A method as claimed in claim 1 in which the maize is selected so that each maize plant develops two, three, four or more ear nodes that are spaced by an internode spacing sufficiently small to ensure that the ears develop substantially synchronously.
 3. A method as claimed in claim 2 in which the maize is selected so that the ear nodes are spaced by an internode spacing of from less than about 50 mm to a maximum of 70 mm.
 4. A method as claimed in claim 1 in which the maize is selected so that the silks develop either at the same time, or before, pollen is released by the tassels of the plants.
 5. A method as claimed in claim 1 in which the maize is a dwarf maize of a prolific plant type grown in high-density populations selected with reference to the availability of photosynthate over the growing season.
 6. A maize plant that, irrespective of its variety, exhibits at least two adjacent shortened internodes spacing at least two ear nodes and a longer internode below the lowermost shortened internode provided that, in the event that the internodes are shortened down to ground level, such longer internode may be absent.
 7. A maize plant as claimed in claim 6 in which the maize plant is selected to develop two, three, four or more ear nodes that are spaced by an internode spacing of from less than about 50 mm to a maximum of 70 mm.
 8. A maize plant as claimed in claim 6 in which the maize plant is selected such that the ears develop substantially synchronously.
 9. A maize plant as claimed in claim 6 in which the maize plant is selected such that the silks develop either at the same time as, or before, pollen is released by the tassels of the plants.
 10. A maize plant as claimed in claim 6 in which the maize plant is both a dwarf maize type and a prolific plant type. 